5 Werkstofftechnik
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Ferritic-austenitic chromia-forming alloys are frequently used as boiler tubes and heat exchanger materials for fossil-,biomass, and co-fired power plants. In all applied environments several strongly corrosive gaseous species such as CO2, SO2, SO3, H2O, O2 exist, causing materials degradation by high-temperature corrosion. The elucidation of degradation mechanisms introduced by multiple gases is challenging due to the presence of different oxidizing agents contributing to the competing reactions for oxidation, sulfurization or carburization. The degradation processes can be divided into initial stages, a transitional stage and the further proceeding steady-state oxidation reaction. Especially the long-term steady-state oxidation and further materials’ life-time are strongly dependent on the initial stages. The adsorption and absorption of the reactive species at the alloy surface and the growing oxide in the initial reaction is further influenced by dissociation and re-reactions of the gas phase molecules. To understand these mechanisms from a fundamental point of view in more detail, dedicated experiments and advanced characterization techniques on various length scale need to be applied. Real-time approaches using highly energetic synchrotron X-ray diffraction showed a high potential to enlighten competitively mechanisms by following the corrosion reactions in-situ in the environment they occur. Despite various other thin film characterization techniques, time of flight secondary ion mass spectroscopy (ToF-SIMS) is a powerful tool to visualize light atoms or labeled isotopes enabling the Differentiation between different oxidizing species. It was especially shown to be applicable in challenging atmospheres containing KCl deposits or in CO/CO2/O2 environments. The present study analyses the competing oxidation/sulfidation process in a humid atmosphere on two ferritic alloys with 2 and 9 % in weight chromium by in situ energy dispersive X-ray diffraction (EDXRD) and comparative tube furnace exposure using S16O2 and H2 18O atmosphere.
The high-entropy alloy (HEA) CrMnFeCoNi and the medium-entropy alloy (MEA) CrCoNi with a face-centered cubic (fcc) structure have excellent mechanical properties and are considered for high-temperature applications. Both materials were exposed under several oxidized atmospheres in a wide temperature range/regime. Mn-oxide (Mn3O4, Mn2O3) was mainly formed on the HEA and Cr2O3 on the MEA. The fast diffusion of manganese prevents the formation of a continuous chromia layer on CrMnFeCoNi in Ar-O2, Ar-H2O and Ar-SO2 at 800°C.
The Cantor alloy and his ternary subsystem CrCoNi were exposed in Ar-O2-H2O-SO2 at 800°C up to 96 h to clarify their oxidation behavior in a mixed atmosphere. The oxidized samples were analyzed by weight measurement, scanning electron microscopy (SEM), and X-ray diffraction analysis (XRD). It was found that mass gain of all samples increased with increasing exposure time. The oxidation rate of the Cantor alloy is significantly higher than that of the ternary alloy system.
Corrosion Science Meets X-Rays, Neutrons and Electrons. The presentation gives an overview on current research activities applying in-situ X-ray diffraction and spectroscopy for a better understanding of fundamental mechanisms of high temperature corrosion. Additionally the knowledge gain by applying neutron powder diffraction and EBSD analysis is presented.